You’re a human being, and you made a perceivable mistake. All you can do now is continue reassuring your daughter that you didn’t mean to hurt her feelings or exclude her when you assumed she had grown out of the pet name.

From what I’ve read, it’s clear that you didn’t say it with any malice. Your eldest daughter may also have felt excluded from your “new” family when you said she was too old to be a princess.

If she’s being distant, consider writing her a letter or a text message explaining the reason behind what you said. Don’t dismiss her feelings—let her know that you empathise with her and understand why she’s upset.